1. Field of the Invention
The present invention relates to an outboard engine structure having an oil pan accommodated inside an extension case.
2. Description of the Related Art
Outboard engine structures with a four-stroke cycle engine mounted thereon are grouped into two types: one having its oil pan formed integrally with one of outer wall defining members of the outboard engine structure such as an extension case; and another having an oil pan formed separately from such outer wall defining member and accommodated inside the latter.
Japanese Utility Model Application Laid-open No. 49199/91 (No. 3-49199) has made known an outboard engine structure of the type wherein an oil pan is formed separate from an extension case and accommodated within the case. In this known engine structure, a recess portion is formed at the rear of the extension case and a drain passage for the oil pan is exposed to the recess portion. Draining of oil is conducted by drawing off a bolt mounted within the recess portion.
When casting an extension case used in an outboard engine structure from aluminum alloy material, generally a casting die is formed of two die parts which are divided into left and right sides. In case of the aforementioned conventional outboard engine structure, however, the recess portion formed at the rear of the extension case has left and right side walls and is opened rearwardly, so that a rear side die part is required in addition to the left and right side die parts, this providing a problem that die parts must be divided in a complicated fashion and hence it results in a high cost.
Moreover, since the recess portion is provided at the rear of the extension case, it cannot be directed toward the ship body side even after steering the outboard engine structure to the maximum so that the operation of draining oil is difficult to be done from an on-board position. Furthermore, if a tool is used to loosen the drain bolt for draining of the oil from the oil pan, the tool may undesirably be contaminated with the oil discharged through a bolt hole.